In recent years, mobile phones or PDAs (Personal Digital Assistants) have been widespread. Also, the specifications in which a compact digital still camera unit or a compact digital video unit is incorporated in the mobile phone or the PDA have been generalized. In a digital apparatus such as the mobile phone or the PDA, a small and low-pixel-number image sensor, as compared with an independent product such as a digital still camera, and an imaging lens device provided with a single focus optical system constituted of one to three plastic lens elements are generally used, in view of severe constraints regarding the size or cost of the digital apparatus.
Since the magnification of the single focus optical system is substantially equivalent to visual magnification, an object to be photographed is limited to the one located near a photographer. Under the current rapid development of high-pixel, high-resolution image sensors, there is a demand for a compact zoom optical system that is compatible with a high-pixel image sensor, and is loadable in a mobile phone or a like device capable of photographing a subject remotely away from a photographer.
For instance, patent document 1 discloses a negative-positive two-component zoom optical system, which is made compact by forming an image-side lens surface closest to the object side into an aspherical shape. In the zoom optical system, since the curvature of the image-side lens surface closest to the object side is large, it is difficult to produce the image-side lens surface by molding. It is required to produce a so-called composite aspherical lens element by coating a resin on a spherical lens element, which may increase the production cost. Also, since production error sensitivity is high with respect to the lens elements in both of the first lens group and the second lens group, an adjustment operation is required at the time of assembling the lens elements, which may further increase the production cost.
Patent document 2 discloses a negative-positive-positive three-component zoom optical system, wherein a moving amount of the lens elements for zooming is suppressed by increasing a zoom load of the second lens group. The optical system disclosed in patent document 2, however, uses three lens elements in each of the first lens group and the second lens group, which is far from sufficient miniaturization.
Patent document 3 discloses a negative-positive-positive-positive four-component zoom optical system, wherein the total thickness of lens elements in a collapsed state is reduced by using an aspherical lens element made of a material having a high refractive index and a low dispersion rate in the second lens group. However, since impact resistance required in a mobile terminal is significantly large, it is difficult to employ a collapsible structure itself. Further, since the optical power of the second lens group in the optical system proposed in patent document 3 is small, a large moving amount is required in the second lens group. Consequently, the entire optical length of the optical system in a use state may be too large for a mobile terminal.
A retrofocus arrangement incorporated with a first lens group having a negative optical power is suitable in an optical system which uses an image sensor and has a zoom ratio of about two to three times, considering general requirements of securing a back focus distance, maintaining a telecentric state, or the like. However, in the case where the retrofocus arrangement is employed, the negative optical power is increased, as the zoom optical system is miniaturized. As a result, the curvature of the lens elements in the first lens group may be unduly increased, which may increase the expansion ratio of the lens elements. Thus, it may be difficult to produce a zoom optical system or measure a surface configuration of lens elements.
Patent document 1: Japanese Unexamined Patent Publication No. 2001-4920
Patent document 2: Japanese Unexamined Patent Publication No. 2001-318311
Patent document 3: Japanese Unexamined Patent Publication No. 2002-365542